The present invention relates in general to a memory card and a method of making the same, and more particularly, to a high-capacity memory card and a method of fabricating the same.
A majority of current digital memory cards is designed based on non-volatile semiconductor flash memory with NAND architecture. Each of the digital memory cards contains one or more flash memory semiconductor integrated circuits (IC) or packages, a controller, some necessary passive components and a printed wiring board substrate (PWB) for interconnection and socket contact. Examples of the digital memory cards include multi-medium cards (MMC), compact flash (CF) cards, secure digital (SD) cards and their modifications such as mini-SD cards and reduced size MMC cards. These cards must conform to specified form factor so that they can fit into various reading slots and readers in various electronics and communications gadgets such as notebook computers, digital cameras, personal computers (PC), and personal data assistant (PDA). For example, an MMC card is typically formed with the dimension of 32 mm×24 mm×1.4 mm.
Currently, the most commonly seen MMC card is in the form of a chip-on-board (COB) assembly, in which a flash IC chip and a controller IC are mounted to a printed wiring board substrate having circuit traces and bond pads formed thereon. The flash IC chip and the controller IC can be electrically connected to the printed circuit board and to each other by bonding wires. The finish bottom side of the substrate is normally coated with a scratch-resistant. Copper/gold plated contact pads are exposed for connecting to a reader device when such MMC card is inserted therein. To further protect the COB assembly, a two-step molding process is performed. The two-step molding process includes applying a transfer mold resin or glob-top liquid encapsulate over the IC's and wiring of the COB assembly, followed by the application of an injection mold resin to form a top portion of the assembly. The overall finish MMC card thickness normally ranges between 1.4±0.1 mm; and for SD card, the over finish thickness normally ranges between 2.1±0.1 mm. In a simplified process, the first step, that is, the application of the encapsulate layer is eliminated and the resin material is directly applied over the COB assembly. Although the simplified process reduces the manufacturing cost, the finished card typically suffers from bowing or bending over a certain period of time due to differences in stiffness and thermal expansion coefficient. FIG. 1 illustrates the comparison between a card 10 with a straight edge and a card 12 that is slightly bowed. As shown, metal contacts 11 are formed one edge of each of the cards 10 and 12. As the metal contacts 11 formed on the card 12 are located on the tapered edge bowing or doming of the card, they are easily stuck in a reader slot or jammed inside the slot and difficult to remove therefrom. Forced withdraw of the card 12 often cause the external label or surface to be scratched. In more severe cases, the card could be damaged or broken.
In another conventional MMC card assembly that includes a TSOP package of a flash IC unit and a controller unit mounted to a printed wiring circuit board substrate, an adhesive material is applied to form a thin cover over the printed wiring board substrate. The assembly includes a plurality of downward gull wing leads soldered to surface-bonding pads of the printed wiring board substrate for interconnecting the TSOP package. The controller unit is connected to the printed wiring board substrate via solder joints. To keep the overall thickness of the assembly within a specified nominal value of 1.4 mm, the top surface of the thin cover has to be minimized. When the top surface cover is made of plastic material, internal voids exist inside the assembly. The very thin top surface sometimes may not provide sufficient mechanical strength to keep the assembly strong and stiff. It is more susceptible to breakage, moisture intrusion, and bending.
It is therefore a substantial need to develop an assembly of a memory card that will retain its original configuration over time and have improved mechanical strength, while the thickness thereof can be minimized by introducing such characteristics.